Removal of porosity in wet-spun acrylonitrile filaments by pressing against a hot surface



Patented May 4, 1954 UNITED STATES .ZPTENT OFFICE REMOVAL OF POROSITY IN WET-SPUN ACRYLONITRILE FILAMEN'I'S BY PRESSING AGAINST A HOT SURFACE N Drawing. Application July 7, 1950, Serial No. 172,630

7 Claims. 1

This invention relates to a process for the preparation of yarns from acrylonitrile polymers, which yarns possess a high tenacity, a desirable elongation, a lustrous appearance, and are substantially free of voids.

Acrylonitrile polymers can be dissolved in certain volatile organic solvents, such as are dis closed in U. S. Patents 2,404,7142,404,727 inclusive, to form solutions suitable for extrusion into shaped articles such as yarns. Wet-spinning processes for the preparation of yarns from these solutions are described in U. S. Patents 2,426,719, 2,451,420 and 2,467,553. These yarns are eminently suited for use in the textile art. In fact, there is a great demand in the textile industry for a yarn having the outstanding properties, such as high tenacity, good outdoor durability, and the insolubility in and insensitivity to common organic solvents, possessed by polyacrylonitrile yarns.

While the wet-spinning techniques described in U. S. 2,426,719, U. S. 2,451,420 and U. S. 2,467,553 yield high tenacity, dense, lustrous yarn from acrylonitrile polymers, the spinning speed and productivity are limited. The dense structures produced are the result of gradual rather than rapid coagulation. A top speed of 100 yds/min. was obtained with glycerol as the coagulating bath. With the less expensive aqueous salt solution as coagulating bath, the spinning speeds were more in the order of 50 yds./min. From the standpoint of production economy, it is desirable to spin a large number of filaments at high rates of speed into an inexpensive coagulating bath, such as water, from which the polymer solvent can be readily recovered. However, this results in yarns having varying degrees of porosity, depending upon the spinning conditions. These porous yarns lack strength and luster, and their use in the textile art is extremely limited.

Eeretoiore, it has been necessary to use nonaqueous baths or aqueous baths containing large amounts of certain salts in order to avoid porosity. Spinning baths of water or substantially water have not proved feasible because of the porosity of the filaments produced. To take advantage of the rapid coagulating action ofsuch baths some means are necessary to avoid or remove the numerous pores or voids.

It is an object of this invention to provide a method in which acrylonitrile polymer yarns are formed in water baths and are converted into yarns that possess a sufficiently high tenacity and elongation to render them suitable for use in the textile art, have a lustrous appearance and are substantially free of voids. A particular object of this invention is to provide a process for spinning into an inexpensive aqueous coagulating bath at high speed and for healing or closing the voids in the resultant acrylonitrile polymer yarns. Other objects will become apparent from the description of the invention which follows.

The objects of this invention are accomplished in general by a process which comprises wetspinning acrylonitrile polymer solutions in water baths or aqueous baths containing a major portion of Water and treating the wet, porous acrylonitrile polymer yarns at a temperature of C. or greater while exerting a lateral pressure on the yarn surface. More specifically, the wet, porous yarn is converted into a substantially solid yarn by passing the yarn under tension over rolls or a plate heated to a temperature exceeding 100 C. until the white, opaque structure becomes clear and translucent.

The ability to heal a yarn, i. e., to convert'a porous yarn to a solid yarn, depends upon the degree-of porosity, which in turn is a function of the spinning conditions, particularly the rate of coagulation. If the rate of coagulation is high, a skin forms on the polymer'structure and the structure cannot collapse properly during coagulation to form a solid yarn. The compactness of the structure can be shown by determining the area ratio of the yarn obtained. The term area ratio refers to the ratio of the measured crosssectional area of the individual filaments of an acrylonitrile polymer yarn to the theoretical cross-sectional. area of those filaments as calculated from the denier of the yarn and the known density of the polymer. When this area ratio is less than 2.1 as is the case when such baths as glycerol baths are used, the voids in the yarn are small and well distributed so that the yarn is healed or becomes completely solid during subsequent normal drawing. The strength of such yarns as expressed in terms of tenacity will generally be in the order of at least 3 g. p. d. and may reach as high as 6.5 g. p. d., depending upon the spinning conditions. As pointed out by'Watkins in U. S. 2,426,719, organic solvent solutions of polyacrylonitrile wet spun into aqueous coagulating baths yield a porous filamentary product and if the area ratio of the porous filaments exceeds 2.1 as is the case when water or baths containing a major portion of Water are used, the yarn has restricted utility. The voids in such a product would be expected to be blown up or enlarged by the application of heat, due to the action of the expanding water. On the contrary, it has been found by the process of this invention that these voids are eliminated by carefully processing the wet, porous polyacrylonitrile yarn under tension on the surface of a roll, plate or pin heated to temperatures of at least 100 C. until the white, opaque structure becomes clear and translucent.

Naturally, the greater degree of porosity, the mor drastic are the processing conditions that are required. This is evidenced by the fact that yarns of medium porosity, i. e., yarns having an area ratio in the range of 2.1 to about 3.3, can be completel and satisfactorily healed by passing the wet yarn through a bath of hot fluid immiscible with the liquid contained by the yarn entering the bath, as described in copending application Serial No. 172,628, filed July '7, 1950. The more porous yarns having an area ratio greater than 3.3 must be submitted to lateral pressure obtained by passing the yarns under tension over a curved plate or around rolls heated to a temperature exceeding 100 C. in order to eliminate the voids. Passage of th porous yarns having an area ratio exceeding 2.1 through air heated above 100 C. without exerting lateral pressure on the yarns will dry the yarns but will not in general cause the elimination of the voids.

The invention may be better understood by reference to the following examples, which are to be construed as illustrative and not limitative and in which parts and percentages are by weight unless otherwise specified.

Example I A solution of parts of acrylonitrile polymer possessing an average molecular weight of approximately 70,000, as determined from viscosity measurements by the Staudinger formula, in 80 parts of dimethyl formamide is extruded at a temperature of 80 C. through a l80-hole spinneret (hole diameter of 0.004") into a spinning bath consisting of 75% water and dimethyl formamide heated to a temperature of 80 C. with the pump delivering polymer solution to the spinneret at the rate of 230 gm./min. The yarn is led through the bath for a total distance of 27", the yarn being subjected during its travel to tension by means of a snubbing tension guide comprising two stationary pins mounted at a distance of 15 from the spinneret face. On leaving the bath the yarn is passed without slippage about a positively driven feed roll possessing a peripheral speed of 100 yds./min., the yarn being subsequently collected on a rotating bobbin possessing a constant peripheral speed of this same value. When dry, this yarn is White and opaque and it possesses an area ratio of .3. Its porosity i readily apparent under the magnification of a microscope. The yarn possesses a dry tenacity of 0.6 g. p. d. and an elongation of 36%. Its loop tenacity is only 0.3 g. p. d.

A sample of this wet yarn was washed free of dimethyl formamide and drawn 5.8 times its original length in a tube in the presence of 30 lbs/sq. in. steam pressure. The wet yarn was then pressed over a series of rolls heated to a temperature of 210 C. and wound up on a bobbin at the rate of 75 yds./min. without further extending it length. The contact time necessary for healing the yarn was 5.0 seconds. When the voids in the yarn are healed, the appearance of the yarn changes from an opaque white to translucent. The resulting 965 denier yarn of 480- filaments had a dry tenacity of 3.9 g. p. d. and

an elongation of 13%. Especially striking was the increase in loop tenacity to 3.2 g. p. d.

Example II A solution of 16.5 parts of polyacrylonitrile possessing an average molecular weight of approximately 10,000 in 83.5 parts of dimethyl formamide was extruded at a temperature of C. through a 480hole spinneret (hole diameter of 0.004) into a bath consisting of 75% water and 25% dimethyl formamide at a temperature of 80 with a pump delivery to the spinneret of 288 gms./m.in. The yarn was led through the bath a distance of 29" and then taken around a submerged bath guide and up to a draw drum operating at a peripheral speed of 234 yds./1nin. The yarn was passed without slippage around this draw drum and then around a squirrel cage piddle drum operating at the same speed, which dropped the yarn by gravity into a collecting can. When dry, this yarn is weal: and brittle and possesses an area ratio of 3.5.

A sample of the wet yarn was washed free of dimethyl formamide and drawn 4.2 times its original length in a tube in the presence of 5 lbs/sq. in. steam pressure. The wet yarn was then passed for a distance of 58" over a curved plate (radius of curvature 50") heated to a temperature of 60 C. The yarn became translucent aiter 15 of contact with the curved plate or in 3.5 seconds. As the healed yarn left the hot plate, it was drawn an additional 1.22 times its length and collected on a bobbin at the rate of 8.3 yds./min. The resulting yarn has physical properties substantially equivalent to those described for the final yarn of Example I.

Example III A solution of 19 parts of a copolymer consisting of acrylonitrile and 5% of Z-Vinylpyridine and having an average molecular weight of approximately 62,000 in 81 parts of dimethyl acetamide was extruded through a l20-hole spinneret (hole diameter of 0.003) into a spinning bath consisting of water heated to a temperature of 90 C. The yarn was led through the bath for a total distance of 59" without snubhing and collected on a perforated bobbin at the rate of 108 yds./min. This 9-denier/filament yarn had an area ratio of 3.6.

A sample of the wet yarn was washed free of dimethyl acetamide and drawn 3.9 times its original length in a tube in the presence of 5 lbs/sq. in. steam pressure. The wet yarn was then passed in spiral fashion around a 6 foot long Calrod of V2" diameter or any similar rod heated to a temperature of C. The yarn made contact with the Calrod for a distance of 39" (one pass around the rod) and became translucent, indicating healing, after a contact distance of 17" or in 5 seconds time. During this heat treatment, the yarn was drawn 1.06 times its length and subsequently collected at the rate of 6.2 yds./min,, being under a total tension of 54 grams. The resulting 2.1-denier/filament yarn had a dry tenacity of 2.8 grams/denier, and elongation of 10%, and a loop tenacity of 2.6 grams/ denier.

Although desirable, it is not necessary to draw the yarn prior to the healing operation of this invention as is the case in the process of copending application Serial No. 172,628. In general, when the wet yarn is not drawn prior to healing, the time of contact with a hot surface to eliminate the porosity is increased. In addition, a

yarn containing solvent cannot be drawn satisfactorily after healing without further conditioning. In order to produce high tenacity yarn, yarn must be drawn several times its original length. With yarns containing solvent, this cannot be'done after healing without conditioning the yarn on the hot surface for an even longer period of time than it took to heal the yarn. Examples IV and V will illustrate this point.

Example IV A bobbin of as-spun polyacrylonitrile yarn prepared in the manner described in Example I and having an area ratio of 3.9 was soaked in a aqueous. dimethyl formamide solution at equilibrium; the yarn was passed through a wringer and led under slight tension around hot roll (139 0.) havin a diameter of 5". The roll speed was 1.7 yds./min. and the wind-up speed was 9.2 yds./min., thereby drawing the yarn after healing 5.5 times its original length. One and a half passes around the hot roll were required to heal the yarn and four and one-half passes were needed to obtain the proper yarn condition for optimum drawing at 139 It took 29 seconds to heal the yarn at that tern perature and another 69 seconds to condition the yarn for good drawing. After this healing and drawing operation, the yarn had a tenacity of 3.9 grams/denier and an elongation of 21%. Its loop tenacity had increased to 3.0 grams/ denier.

Example V A solution of 21 parts of a copolymer containing 95% acrylonitrile and 5% of 2-vinylpyridine and possessing an average molecular weight of approximately 62,000 in 79 parts of dimethyl formamide was extruded at a temperature of 80 C. through a 480-hole spinneret (hole diameter of 0.003") into a spinning bath consisting of 77% water and 23% dimethyl formamide heated to a temperature of 68 C. with the pump delivering polymer solution to the spinneret at the rate of 293 grams/minute. The yarn was led through the bath for a total distance of 53", the yarn being subjected during its travel to tensionby means of a snubbing tension guide mounted at a distance of 12" from the spinneret face. The Q-denier/filament yarn pro duced was collected on a perforated bobbin at the rate of 162 yds./min. and possessed an area ratio of 3.5.

This yarn was washed free of dimethyl form-- amide and while still wet given three passes around the 6 foot Calrod of diameter heated to a temperature of 167 C. The yarn was passed in spiral fashion around the Calrcd so that its total distance of contact with the rod was 53". The elimination of porosity as evidenced by transluc nt appearance took place after 37" of contact or after 22 seconds. Since the yarn could not be drawn more than 1.7 times its length from the Calrod, the wind-up speed was 4.7 yds./min. While the yarn healed very well, its tenacity was only 1.3 grams/denier at l5% elongation. This tenacity can be raised to the neighborhood of 4 grams/denier by means of a subsequent hot roll drawing process. 7

It is essential in the operation of this inven tion that the yarn being healed be thoroughly wet. The healing takes place only while the yarn is drying. It is not essential that water be the wetting liquor but water certainly is the easiest to handle and the most economical.

6. Polar substances similar to water and having relatively low boiling points may be used such as the alcohols like ethyl alcohol, butyl alcohol, dioxane, etc. Water or aqueous mixtures are preferred. It is thought that the healing actually takes place because a slight vacuum is developed as the vaporized liquor leaves the yarn and this vacuum in addition to the slight lateral pressure supplied collapses the porous structure.

In preparing the shaped article, the polymer can be dissolved in any suitable solvent. However, it is preferred that the solvent comprises a volatile organic substance. The term volatile organic solvent as used in this specification is meant to include those organic substances that are capable of forming stable homogeneous solutions with an acrylonitrile polymer and that can be distilled without decomposition at atmospheric pressure. This class of substances is exemplified in U. .5. Patent Nos. 2,404,7l4- 2,404,727, inclusive.

The preferred concentration of the spinning solution is, of course, dependent upon the average molecular weight of the polymer employed, it generally being desirable to use more dilute solutions in the spinning of polymers of high molec ular weight. It has been found that the viscosity of the spinning solution has an appreciable effect on the degree of porosity of the spun yarn. Solutions of acrylonitrile polymers in dimethyl formamide having a viscosity of from 30-60 poises at 125 C. result in the formation of the least porous yarns when spun into aqueous baths, particularly at spinning speeds in the range of -200 yds./min.

Since the severity of the healing operation required depends upon the degree of porosity of the as-spun yarn, it is naturally prefererd to prepare yarns having a minimum as-spun porosity. When spinning acrylonitrile polymer solutions into aqueous coagulating baths at speeds of 100 yds./min. and higher, the porosity is also minimized by submitting the freshly coagulated yarn to a snubbing tension in the coagulating bath. The snubbing tension guides may be placed be tween 4 and 15" from the spinneret face in order to minimize the porosity. If the snubbing tension guides are placed more than 15" from the spinneret face, the degree of porosity is not affected but the size of the pores is decreased. This is similar to the efiect obtained by drawing the as-spun yarn. While porous yarns obtained without snubbing may be healed in accordance with this invention, it is preferred to use snub bing tensions to reduce porosity and facilitate healing.

The coagulating baths used may be water or aqueous solutions which gel the yarn structures rapidly. Otherwise, the spinning speed must be low and productivity is limited. The preferred coagulating baths are aqueous solutions containing from 20-50% of the polymer solvent being used, although the amount of water may be lower, as, for example, 35%. The solvent can be readily recovered. from such aqueous solutions and spinning speeds up to 300 yds./min. can be used with-- out developing more porosity than can be handied. The temperature of the coagulating bath is preferably in the range of 50-l00 (7., although other temperatures may be employed. The bath. temperature should, of course, not exceed the boiling point of the solvents employed in the spinning solution. While porous yarns from any pin ning bath or the like may be healed as described herein, this invention is particularly useful in connection with the economical aqueous baths.

It is generally preferred to employ a bath travel of the order of 20-60. However, this is not essential and bath travels as short as 6" can be used if desired. One of the advantages of this invention is that large bundles of filaments intended for conversion into staple fibers can be readily spun into the relatively short aqueous coagulating baths. The porosity developed in such a process can be readily healed by the process of this invention. In the preparation of yarns having an area ratio of less than 2.1, as described in U. S. 2,426,719 and U. S. 2,451,420, it is indicated that much longer bath travels of the order of 100-200 inches are to be used in the spinning of heavy denier yarn or large bundles of filaments.

As indicated earlier in this specification, the highly porous yarns, e. g., those Whose area ratio is greater than 3.3, must be passed under tension over a curved hot surface in order to completely eliminate the voids. Two slightly differing methods for accomplishing this are described in the examples. The process described in Examples I-III is to be preferred. The solvents for acrylonitrile pclymers containing at least 85% acrylonitrile may be toxic or have an irritating odor. For this reason. it is preferred to wash the solvent completely from the yarn before the hot healing operation. This may be done continuously immediately following coagulation or in a subsequent operation. In the case of highly porous yarns completely free from solvents, it is desirable to minimize the size of the voids by a drawing operation prior to healing. Drawing from 1.5-2 times the original length in an atmosphere of steam is generally satisfactory. The highly porou drawn yarns free from solvents heal very rapidly in the subsequent heat treatment of this invention. The time of contact for healing such yarns is generally less than 10 seconds. This is due in part to the much higher surface temperatures that can be used in the absence of a solvent for the yarn. If high tenacity yarns are desired, the yarn may be further oriented by drawing at the time of healing. On the other hand, all of the draw may be applied in the steam drawing operation prior to healing and, in this case, steam pressures as high as 4050 lbs/sq. in. are generaly used.

As shown in Examples IV and V, highly porous yarn may be completely healed without first drawing the yarn. If the process of Example IV is used, it is preferred that the solvent in the yarn makes up from 20-35% of the total liquid retained by the yarn after removing the excess from the surface. Such yarns will heal uniformly and completely without sticking of filaments on rolls heated to temperatures in the range of 130160 C. The healing time, that is the time at which the yarn becomes translucent on the roll, is generally in the neighborhood of 30 seconds. However, the yarns must be kept in contact with the hot surface for at least an equal length of time after healing in order to draw them properly. The healing takes place with the removal of the water, but the yarn at this point still retains too much solvent to be drawn properly. This can be illustrated by a slight variation in the method of Example IV. If the as-spun yarn is drawn 4 times its length in steam without removing the solvent from the yarn, the yarn may be subsequently healed on the hot roll in 12 seconds, but a total contact time of about 62 second was nevertheless necessary before it could be satisfactorily drawn further.

The lateral pressure exerted on the yarn to get healing is for convenience measured by tension. Generally, the tension is less than 0.1 g. p. d. It may be lower or higher, however, depending upon the conditions of healing. For example, when a curved surface of -inch radius is being used, satisfactory healing was obtained using a tension of 0.14 g. p. d. on a yarn having an initial porosity of 4.1. With a rod of 56-inch radius, a tension of 0.09 g. p. d. yas satisfactory on a yarn of 3.5 initial porosity ratio. For most purposes a tension between 0.05 and 0.2 g. p. d. will work. The time may be very short as, for example, 3 seconds contact with the hot surface up to 20 seconds. The time may vary from this somewhat, of course, depending upon the temperature of the surface, the thickness of the yarn, etc.

If the liquids content of the yarn or the solvent/water ratio is too low, higher temperatures are required and the healing is generally less uniform. In these cases, added pressure applied to the yarn on the hot roll at one point before healing will improve the healing greatly but stuck filaments will generally result. Higher solvent content, such as 50%, based on the total liquid retained by the yarn, readily gives perfect healing but also severe stuck filaments. It can be seen that good yarns are readily obtained by these methods but that the contact time for good drawing is over a minute. Another disadvantage rests in the ventilation necessary in the area surrounding the hot rolls to take care of the high concentration of solvent vapor.

As pointed out earlier in this specification, the more porous yarns require more drastic healing conditions. The lateral pressure exerted on a yarn being healed may be increased by decreasing the radius of curvature of the hot surface.

. It will be apparent that in the use of heated rolls for healing yarn, the lateral pressure sustained by the filaments during passage around the rolls will be related to the radius of curvature of the rolls and the angle of contact with the curved surface. For example, a change of direction of around a small roll will in general apply more pressure laterally to the filaments than a similar passage around a large roll. This is due to the fact that the tension of the yarn is carried by a greater length of roll surface in the latter case. In the event that a heated rod is used for healing the yarn, the lateral pressure on the yarn may be readily varied by changing the number of passes around the rod per unit length of rod.

The process of this invention makes it possible to convert dull, porous and weak acrylonitrile polymer yarns into strong lustrous yarns substantially free of voids. The process can be operated on standard equipment and broadens the utility of cheaply prepared yarns containing a major portion of acrylonitrile.

Any departure from the above description which conforms to the present invention is intended to be included within the scope of the claims.

I claim:

1. A process of converting a porous filamentary article of an acrylonitrile polymer containing a major portion of acrylonitrile to a substantially non-porous condition, which comprises subjecting said article wet with water to lateral pres-- sure against a solid surface at a temperature below the thermal decomposition point of said polymer and at least 100 C. until said water is removed and said article is substantially nonporous. j

2. A process for converting a porous yarn of an acrylonitrile polymer containing a major portion of acrylonitrile to a substantially non-porous yarn which comprises wetting said yarn with water; placing a tension of from 0.05 to 0.2 gram per denier on the resultant wet yarn; and pressing the tensioned yarn in air against a hot solid surface heated to a temperature below the thermal decomposition point of said polymer and at least 100 C. until the said yarn is "substantially non-porous.

3. A process for converting a porous yarn of an acrylonitrile polymer containing a major portion of acrylonitrile to a substantially non-porous yarn which consists of stretching said yarn; wetting said yarn with liquid water; placing a tension of from 0.05 to 0.2 gram per denier on the resultant wet yarn; and pressing the tensioned yarn in air slidably against a hot solid surface heated to a temperature below the thermal decomposition point of said polymer and at least 100 C. until said yarn is substantially non-porous.

4. A process for converting articlesflshaped as yarns, films, and ribbons from an acrylonitrile polymer containing a major portion or acrylonitrile and having an area ratio or at least 3.3 owing to the presence of voids to substantially non-porous shaped articles which comprises wetting the said articles with liquid water and pressing the resultant wet article in air against a hot solid surface heated to a temperature of about 100 C. to about 210 C. until the said water is removed and the area ratio is reduced to less than 2.1 owing to substantial collapse of the voids.

5. The process of claim 2 in which said polymer is the homopolymer polyacrylonitrile.

6. The process of claim 2 in which said polymer is a copolymer of acrylonitrile and a vinylpyridine.

7. The process of claim 6 in which said vinylpyridine is 2-viny1pyridine.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,035,190 Renfrew Mar. 24, 1936 2,282,568 Finzel May 12, 1942 2,353,270 Rugeley et a1 July 11, 1944 2,365,931 Benger Dec. 26, 1944 2,369,057 Leary Feb. 6, 1945 2,394,540 Finzel Feb. 12, 1946 2,395,396 Conaway Feb. 26, 1946 2,420,565 Rugeley et a1 May 13, 1947 2,445,042 Silverman July 13, 1948 

1. A PROCESS OF CONVERTING A POROUS FILAMENTARY ARTICLE OF AN ACRYLONITRILE POLYMER CONTAINING A MAJOR PORTION OF ACRYLONITRILE TO A SUBTANTIALLY NON-POROUS CONDITION, WHICH COMPRISES SUBJECTING SAID ARTICLE WET WITH WATER TO LATERAL PRESSURE AGAINST A SOLID SURFACE AT A TEMPERATURE BELOW THE THERMAL DECOMPOSITION POINT OF SAID POLYMER AND AT LEAST 100* C. UNTIL SAID WATER IS REMOVED AND SAID ARTICLE IS SUBSTANTIALLY NONPOROUS. 